Overrunning clutches are widely used, especially in automatic transmissions, to allow selective relative rotation between a pair of inner and outer coaxial clutch races. One clutch race has a cylindrical surface, which is generally referred to as the pathway, so the race itself may be referred to variously as the ring, pathway, or pathway race. The other race has an inner surface that includes a series of evenly circumferentially spaced and sloped cam ramps. Either the inner or the outer race may be the cam race. In either case, the cam ramps face the pathway, creating wedging pockets with a narrow and a wide end. A clutch cage is installed between the races, tied to and carried by the cam race. The cage retains a plurality of cylindrical rollers, locating one in each wedging pocket. Each roller is continually urged by an energizing spring toward the narrow end of the wedging pocket, and is thereby kept in continual contact with both the cam ramp and the pathway, which may termed the roller ready position.
In the transmission environment, various forces on the races will tend to rotate the races at various directions and speeds relative to ground, and in various directions relative to one another. However, given the continual spring bias of the rollers toward the narrow end of the wedging pocket, only selected relative rotation between the clutch races is possible. If the races tend to rotate in a relative direction that moves the rollers toward the wedging pockets' narrow end, they jam quickly between the pathway and cam ramps, and the races lock up. If the races tend to rotate in the opposite relative direction, that is, tending to move the rollers toward the wide end of the wedging pockets, the rollers slip between the pathway and the cam ramps, and the races can freely overrun. Theoretically, lock up and overrun depend only on the relative directions between the races, unaffected by the absolute speeds that the races are experiencing, that is, their speeds relative to ground. The absolute race speeds may vary widely from environment to environment. Likewise, theoretically, lock up and overrun do not depend on whether the outer or inner race is the cam race. As a practical matter, however, the behavior of the rollers is affected significantly by whether the roller carrying cam race is the outer or inner race, and by whether the absolute speed experienced by the cam race during overrun is small or great.
For example, in the case where the outer race is the cam race, if the outer cam race is also experiencing high speed during overrun, then the rollers are thrown centrifugally outwardly, tending to move away from the pathway, a phenomenon known as roller drift. If severe enough, the rollers may overpower the energizing springs and actually move physically away from the pathway, threatening the roller ready position necessary for quick lock up. The clutch shown in U.S. Pat. No. 4,549,638 to Johnston, assigned to the assignee of the present invention, is a response to the roller drift problem. In Johnston, the springs are mounted on weighted sliding blocks that shift opposite to the direction of roller drift, increasing the spring pressure on the rollers. If the outer cam race is not seeing high absolute speeds during overrun, roller drift is not a problem, but a different problem may manifest itself. The springs continually lightly load the rollers into the narrow end of the wedging pockets, against both the pathway and the cam ramps. Despite the light spring force, the pathway may be moving very rapidly relatively to the cam ramps. Traction of the pathway on the roller can cause the rollers to spin, rather than just slip on the pathway. As the roller changes from pure slipping to spinning, evenly distributed wear on the pathway is traded for potential localized wear on the cam ramps. This localized wear can affect the operation of the clutch by effectively changing the angle of slope of the cam ramps, known as the strut angle.
In the case where the inner race is the cam race, and the cam race is also seeing significant absolute speed during overrun, centrifugal force acts to throw the rollers even more strongly into the pathway, increasing the potential traction on the rollers. If the inner cam race does not see significant speed during overrun, centrifugal force won't increase traction. However, the energizing spring force can still cause pathway traction and roller spin, although it would not be as severe. So, to summarize, roller spin during overrun is a potential problem in every case, except where roller drift is significant enough to lessen the pathway traction.
The rollers do not slip, or spin, during lock up, of course. However, as the races attempt to reverse relative direction, changing from overrun to lockup, the rollers will be forced very strongly and very quickly toward the narrow end of the wedge. This can actually forcibly open the narrow end of the wedge a small amount. As this happens, the rollers will roll up the cam ramps a few degrees in the direction opposite to the direction in which they were spinning during overrun, a process known as roller windup. Although the rollers experience a lot of load during windup, they do not cause significant wear on the races, because they can roll freely.
The prior art recognizes the overrun roller spin and wear problem, in at least one instance. But the solution proposed there does not work well when it is most needed, that is, in the case where the inner race is the cam race and is moving fast during overrun. Furthermore, when the proposed solution would work well, it isn't needed in the first place, that is, in the case where the outer race is the cam race, and is moving fast during overrun. U.S. Pat. No. 2,044,197 to Barthel proposes biasing the rollers to ready position indirectly, by weighted blocks pivoted to the cam race in such a way as to pivot away from the rollers and reduce the spring pressure when the cam race is rotating at high speed. In its FIG. 1 embodiment, where the inner race is the cam race, the spring pressure would in fact be reduced, but nothing would prevent centrifugal force from throwing the rollers out into the pathway. In its FIG. 3 embodiment, where the outer race is the cam race, the spring pressure would also be reduced, but roller drift would already be acting to reduce pathway traction, anyway. And, in either embodiment, if the cam race were not moving at high speed, there would be no effect on spring pressure at all, as nothing would act to pivot the weights. In addition, the pivoted weights take up so much room between the races that the energizing springs are too small to be practical.